The present invention relates generally to power hand tools such as drivers for applying torque to a fastener, or drills for use with rotary cutting tools, in which the output torque is user-adjustable.
It is known to provide power tools with a mode-selector that the operator uses to change between a driver mode in which a torque limit may be selected for driving fasteners, and a drill mode in which no torque limit is set. The mode selector may simultaneously change between a high-torque, low-speed operation in the driver mode, to low-torque, high-speed in the drill mode, as by connection of the mode selector to a speed-change gearbox.
In a commonly available tool, a torque limiting clutch is used and, by varying the pre-load on a clutch spring, the torque at which the driving member slips relative to the driven member is controlled. A control sleeve may be mounted about the spindle at one end the tool housing (for instance, adjacent the chuck) for making the torque selection. Such an arrangement is ergonomically advantageous, as the tool can be supported for use and for rotation of the sleeve generally about the spindle axis to vary the torque selection, without the need loosen a grip on the tool. The clutch is disposed between the gearbox and the output end of the spindle, of the chuck, so one disadvantage of this arrangement is that it extends the axial dimension of the tool. It will be understood, therefore, that there is a need for a tool which is relatively more compact, allowing for use in tighter spaces.
As an alternative to a clutch in a power tool, it is known for instance in tools like that described in DE3103286, to employ a potentiometer by which a torque limit value can be set. In such tools, a control circuit includes a switching element for controlling current flow to the motor, and which compares the level the current consumed by the motor with a current value set by means of the potentiometer. In operation when the current drawn by the motor reaches the value set by the potentiometer, and thus the selected maximum value of the torque, the switching means cuts power to the motor. However, to date there remains an unmet need for a compact power tool with an ergonomically efficient arrangement that allows the power tool to be manufactured cost effectively. It is an object of the present invention to address these needs or, more generally, to provide an improved power tool.